1. Field of the Invention
The present invention rates to an improved method for the purification and concentration of soy protein, and particularly to a method in which genetically-modified soybeans are used to produce an improved soy protein concentrate.
2. Background of the Art
The soybean, Glycine max, is a leguminous crop grown in many parts of the world. Soybeans are of great economic importance as a source of edible oil, high-protein foods, food ingredients, and stockfeed, as well as many industrial products. Native to Eastern Asia, the soybean has been used as the chief source of protein for millions of people in the Orient for centuries. It was not until the late 19th century, however, that soybeans began to attract serious attention from Western scientists.
Initially, the primary market for soybeans in the Western World was industrial, causing high demand for the oil to make plastics, adhesives, textile fibers, paper coatings and fire-fighting foams. Later, the demand grew for high-quality soybean meal for use in various animal feeds such as poultry and swine. (Soybean meal is the high-protein residue from the extraction of soybean oil). By the 1930's, the Western market for soy protein substitutes for human consumption began emerging. The United States is now the world's leading producer of soybeans, producing 60-65% of the world crop, with over 56 million acres of soybeans grown each year in the United States. Brazil, China, and Argentina together account for about 33% of the world's production.
Starting in the 1950's, the Western market for soy protein products for human consumption grew tremendously. Today, soybeans are the most abundant source of vegetable protein and oil in the world. The soybean crop of the United States provides over $500 million/year of edible protein products. Soybeans are an important food because they are unusually complete in proteins, with excellent amino acid balance. Soybeans can be eaten as a green or dried bean, and as a constituent in many products including beverages, infant formula, cereals, baked goods, curds, cheese, various sauces, and so forth. Increasingly, soy protein is also being used as a meat substitute.
The term "soy proteins" typically refers to processed, edible dry soybean products other than soybean meals for animals. The three most important considerations in soy protein production are the functionality of the final product, the amount of indigestible oligosaccharides (raffinose and stachyose) present, and the palatability or flavor of the soy protein product. Also affecting the usage of the final product is the amount of fiber present.
Functionality refers to the characteristics which affect protein behavior in various foods during processing, manufacturing, storage and preparation. The level of functionality of a soy protein product is determined by many factors, but in general terms is directly related to the amount of soluble proteins present. Although an insolubilized protein contains basically the same functional groups as a soluble or "native" protein, there is a difference in the "accessibility" of these reactive groups. Hydration is still possible with insolubilized protein, but the resulting product is a suspension rather than a solution. Fat and oil can also be absorbed by insoluble protein, but to a lesser degree than water. Some soy proteins are texturized to absorb fat and produce meat-like texture.
Functional properties of soy protein products include solubility, foaming, emulsification, water absorption and binding, viscosity, gelation, cohesion-adhesion, elasticity, fat absorption, flavor-binding, color control, thermoplasticity, and the ability to form edible films. Depending on the desired final food product, the various functional properties have varying degrees of importance. Successful incorporation of soy proteins into traditional food products such as meat, poultry, seafood, eggs, and dairy products, typically requires the protein ingredient to exhibit characteristics similar to those of the protein being replaced or supplemented.
"Soy Protein Products" of the Soy Protein Council and Table 8.8 in Chapter 8, page 152 of the "Practical Handbook of Soybean Processing and Utilization" by D. R. Erickson each discuss functional properties, mode of action, food systems in which those properties are used and products using those properties.
As stated above, water solubility is considered an index of the functional properties of a soy protein as well as an index of the biological and enzymatic activities. Water solubility in soy proteins can be measured by the Nitrogen Solubility Index (NSI). Solubility is important in milk and various other beverages as well as infant formula and liquid animal feed. A highly functional or highly soluble protein product typically has an NSI greater than about 85%. A moderately functional protein product has an NSI of about 60% to about less than 85%, while a poorly functional protein has an NSI less than about 60%.
As with solubility, the various other functional properties indicate the composition of the proteins as well as their interaction with other food components. For example, foaming refers to the ability of the product to form films or to entrap gas and is measured by volume of foam per unit of protein. This function is important in whipped toppings, chiffon desserts and angel cakes. Emulsification is the ability to form and stabilize fat emulsions and is measured by the emulsive capacity of the oil emulsion per gram of protein. This is a quantitative measurement of the ability of the material to support a stable emulsion under identified and standard conditions. This function is required in foods such as sausages, bologna, soups, meat pumping solutions and cakes.
Functionality is also affected by the method used to make a particular product. For example, a soy protein concentrate made with alcohol leaching has a much lower NSI and therefore a lower functionality than a soy protein concentrate made using acid. This is because the protein has been denatured to a greater extent by the alcohol and heat used in the alcohol leaching process. Functionalities may also be modified and improved by adjustment of pH with sodium or calcium bases, application of mechanical stress, and hydrolysis by proteolytic enzymes before drying. Other steps which affect functionality include jet cooking or high pressure homogenization which increase the NSI.
The oligosaccharides raffinose and stachyose are present in soybeans. The concentration of these oligosaccharides are an important consideration in soy protein products because humans and other monogastric animals have difficulty digesting these naturally-occurring carbohydrates. Raffinose is present naturally in soybeans in concentrations of less than one (1)%, while stachyose is present in concentrations of about three (3) to five (5)% by total weight of the dry soybean.
Specifically, monogastric animals lack the enzyme "alpha-galactosidase" necessary to hydrolyze the "alpha-galactosyl" linkages present in raffinose and stachyose to simpler sugars which are absorbable. Instead, the compounds enter the lower intestinal tract fully intact, where they are metabolized by bacteria and actually ferment to produce flatulence and intestinal gas. As a result, no digestible nutrients are obtained and considerable discomfort can result. Many scientists also believe that the presence of oligosaccharides reduces feed efficiency as well.
Palatability or flavor is an important consideration because the natural flavor of a soybean is considered to be an off-flavor or beany-flavor due primarily to the activity of the enzyme lipoxygenase. Lipoxygenase is comprised of three isoenzymes known as lipoxygenase-1, lipoxygenase-2, and lipoxygenase-3. Research is ongoing to determine if lipoxygenase activity is the sole source of the off-flavor or whether other enzymes or flavor compounds in soybeans contribute as well.
It is clear, however, that the "beany" or "painty" flavor of soy proteins is due in large part to lipoxygenase-catalyzed oxidation of linoleic and linolenic fatty acids in the oil. Lipoxygenase is active in the presence of moisture, and unless deactivated will cause obnoxious flavors and odors. Current processes may partially deactivate the lipoxygenase by heating the soybeans. Furthermore, the alcohol used in the alcohol-leaching process to produce soy protein concentrates actually extracts some of the products of the oxidation reaction involving lipoxygenase. In extremely small amounts, however, lipoxygenase is used to bleach bread by the active lipoxygenase action on wheat carotenoids.
Another important consideration in soy protein products is the amount of non-functional fiber present in the final product. Natural soy fiber is derived from the parenchyma cell walls of the soybean. The presence of fiber can bind up the soluble protein, and also prevents a soy product from being used in various applications. Although diets high in fiber are known to have certain nutritional advantages, there are many applications in which fiber is not desirable.
There are numerous other considerations relevant to composition and nutrient content of soy protein products. Such considerations include the presence of trypsin inhibitors which inhibit the digestion of proteins and hemagglutinins (lectins), and which must be heat-inactivated as they otherwise exert negative effects on the nutritional quality of soybean protein. Phytins, which are the insoluble magnesium-, calcium-, and potassium-complexed salt of phytic acid, can bind up protein in the final product. Soybeans also contain goitrogens, the antivitamins D, E and B12, as well as isoflavones, phytoestrogens, saponins, vitamins, minerals, and so forth. Research continues regarding other nutritional and biochemical effects of the soybean which include its cholesterol-lowering ability and anticarcinogenesis.
Soy protein products for human consumption fall into three major groups: Soy flours and grits having 52 to 54% Protein (N.times.6.25) on a moisture-free basis (mfb), soy protein concentrates containing at least 65% Protein (N.times.6.25) mfb, and soy protein isolates (or soy proteinates) having a minimum of 90% Protein (N.times.6.25) mfb. The term "% Protein (N.times.6.25)" is often used to express the percent of protein in soy protein products to reflect that only part of the nitrogen in soy proteins is of protein origin. The American Oil Chemists' Society (AOCS) conversion factor for soybean protein is N.times.5.71; however, industry practice is to label protein in soybeans as "Protein (N.times.6.25)."
Soy flours and grits are the least refined forms of soy protein products used for human consumption and may vary in fat content, particle size, and degree of heat treatment. These products also still contain about five (5) to six (6)% of the oligosaccharides, most of the original lipoxygenase, as well as about 4.3% fiber. As a result, they can only be used in small amounts in various products as otherwise intestinal discomfort and poor flavor become overriding considerations. Soy flours and grits are considered to be "poorly" functional and typically have an NSI less than about 60%.
Soy protein concentrates have much of the indigestible oligosaccharides removed such that the raffinose content is less than about 0.5% and the stachyose content is less than about three (3)%. However, depending on the process used, soy protein concentrates have only poor to adequate flavor, and low to adequate functionality, having NSI's in the range of 15-70%. Additionally, the various processes for producing soy protein concentrates result in a recovery of only about 50% to about 95% of the protein. In every instance, the high cost of such processes limit the use of these products in many areas such as aquaculture diets, poultry diets, and so forth. Furthermore, the presence of approximately four (4)% fiber in soy protein concentrates make them unsuitable for use in certain products such as beverages, milk, and infant formula. The current processes also remove important minerals, vitamins, isoflavones and phytoestrogens along with the low molecular-weight sugars, ash, and minor components.
Soy protein isolates are the most highly refined soy protein products commercially available as well as the most expensive. As with the soy protein concentrates, soy protein isolates are also low in oligosaccharides, having negligible amounts of raffinose and less than two 2(%) stachyose in the final product. Additionally, the isolates have a satisfactory flavor and are highly functional, having a NSI in the range greater than about 85%. Isolates also improve dispersibility and reduce dusting. Both gelling and non-gelling varieties are available as well as various viscosity grades. They possess a low fiber content of less than about 0.3%. As discussed above, it is desirable to remove the fiber in certain products because fiber is non-functional and dilutes protein content. However, as with soy protein concentrates, many of the valuable minerals, vitamins, isoflavones, and phytoestrogens are drawn off to form a waste stream along with the low-molecular weight sugars in making the isolates. Also, the process for producing soy protein isolates yields only about 33% to 50% solids recovery and about 62% recovery of available protein. This extremely low yield along with the many required processing steps contributes to the high costs involved in producing soy protein isolates. As such, their use is prohibitive in many lower value applications including fish food, poultry feed, pet foods, weaning feeds for baby animals, etc.
Soy protein isolates containing a relatively high amount of albumins and other acid soluble proteins have been produced in the laboratory as described in U.S. Pat. No. 4,863,613 to Johnson et al. This patent discloses that the use of absorbable gels results in an economical process for producing soy protein isolates since no acids or alkalis are used and no centrifugation is necessary. However, such a process is not commercially viable not only because the required gel volume for a large-scale operation would be extremely large and cumbersome, but also because their gels have to be regularly replaced, produce a useless by-product, and would otherwise be cost inefficient.
Thus, what is needed is a low-cost and practical method for commercially producing an improved soy protein concentrate having low oligosaccharide content, minimal lipoxygenase activity and/or good flavor, as well as high functionality. Additionally, a soy protein concentrate is needed which retains most of the naturally-occurring vitamins, minerals and isoflavones, but contains little or no fiber.